This invention relates to the field of back-lighted copyboards used in process cameras. In process cameras subjects to be photographed, such as the layout of an advertisement, are positioned upon an illuminated copyboard and the image thereof is projected by a lens system upon a photosensitive medium to produce positive or negative stats, which in turn may be employed to produce printing plates. Copyboard lighting methods intended to correct the problems engendered by the cosine.sup.4 law are old in the art. For example, in the U.S. Pat. No. 3,375,752, issued to Henry Fairbanks and Thomas Madigan, Jr., a copyboard illumination system is disclosed which compensates for the cosine.sup.4 law for wide-angle camera lenses. The cosine.sup.4 law holds that the intensity of the object off the axis of the lens is decreased by a function of the cosine.sup.4 of the angle of the object from the lens axis, which in turn requires that the edges of the copy be more brightly illuminated than the center, to obtain an image of even intensity. In the aforesaid patent, special light fixtures are provided over the copyboard for producing higher illumination intensities of the noncentral portions of the copyboard. The detrimental effects of this law are also described in U.S. Pat. No. 3,519,346, issued to Daniel H. Robbins.
It is an object of the present invention, unlike above mentioned patents, to provide a back-lighted copyboard which employs a special unique light attenuator, which may be inexpensively manufactured, and which produces consistent results in terms of eliminating the detrimental effect of the cosine.sup.4 law. It is also desirable to back-light the copyboard by means of elongated fluorescent lamps which inherently produce unequal illumination due to the inverse square law and due to an inherent reduction in the intensity of light generated at end portions of the elongated lamps.
It is thus an object of the present invention to employ a special light attenuator which compensates for both cosine.sup.4 law characteristics of a projection lens and at the same time, compensates for the above mentioned unequal intensity produced upon a light diffuser positioned over a bank of elongated lamps due to the elongated nature of such lamps.